Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/unicode-match-property-value-ecmascript/index.js

@@ -0,0 +1,19 @@
+'use strict';
+
+const propertyToValueAliases = require('./data/mappings.js');
+
+const matchPropertyValue = function(property, value) {
+	const aliasToValue = propertyToValueAliases.get(property);
+	if (!aliasToValue) {
+		throw new Error(`Unknown property \`${ property }\`.`);
+	}
+	const canonicalValue = aliasToValue.get(value);
+	if (canonicalValue) {
+		return canonicalValue;
+	}
+	throw new Error(
+		`Unknown value \`${ value }\` for property \`${ property }\`.`
+	);
+};
+
+module.exports = matchPropertyValue;